Aqueous metallic coating compositions containing a metallic pigment such as an aluminum pigment have a problem in that the metallic pigment comes into contact with a large amount of water and reacts therewith to generate hydrogen gas.
To solve this problem, incorporation of a phosphoric ester into the aqueous metallic coating composition has been proposed to protect the metallic pigment surface with a phosphoric ester by the adsorbing action of acidic hydroxy groups bonded to the phosphorus atoms in the ester, thereby inhibiting the reaction caused by direct contact between water and the metallic pigment.
For example, Japanese Unexamined Patent Publication No. 1987-30167 discloses an aqueous metallic coating composition comprising: an acrylic resin having a hydroxy value of 30 to 150 mg KOH/g and obtained by copolymerizing a specific phosphate group-containing monomer with an α,β-ethylenic unsaturated monomer; a curing agent; and a metallic pigment. The publication describes that the aqueous metallic coating composition can form a coating film with excellent water resistance against discoloration and excellent metallic appearance. However, the coating film formed using the metallic coating composition is insufficient in terms of smoothness and water-resistant adhesion.
U.S. Pat. No. 5,151,125 discloses an aqueous metallic coating composition comprising: an acrylic resin having an acid value of 15 to 200 mg KOH/g and a hydroxy value of 20 to 200 mg KOH/g and prepared by copolymerizing an ethylenic monomer and a phosphoric acid group-containing monomer having an alkylene oxide chain; a curing agent; and a metallic pigment. The patent describes that the aqueous metallic coating composition can form a coating film with high adhesion and excellent water resistance against discoloration. However, the coating film formed using the aqueous metallic coating composition is insufficient in terms of smoothness, luster, and water-resistant adhesion.